Deprecate support for Ruby versions earlier than v2.0

This is long overdue. Extended maintenance of Ruby v1.9.3 ended on 23
Feb 2015 [1]. Actually dropping support will allow significant
simplification of the code.

The immediate motivation is to come up with better parameter matching
for keyword arguments and we've run into parsing issues when trying to
do this in conjunction with continuing to support Ruby v1.9.

See #325.

[1]: https://www.ruby-lang.org/en/news/2014/01/10/ruby-1-9-3-will-end-on-2015/

lib/mocha/ruby_version.rb

@@ -1,3 +1,11 @@
+require 'mocha/deprecation'
+
 module Mocha
   RUBY_V2_PLUS = Gem::Version.new(RUBY_VERSION.dup) >= Gem::Version.new('2')
+
+  unless RUBY_V2_PLUS
+    Mocha::Deprecation.warning(
+      'Versions of Ruby earlier than v2.0 will not be supported in future versions of Mocha.'
+    )
+  end
 end